Dental restorative material

ABSTRACT

A dental restorative composition is provided that includes a polymerizable resin, a substantially translucent structural filler, a nanofiller having a mean particle size less than 100 nm, and at least one rheology-modifying additive. In one embodiment, the structural filler has a refractive index substantially similar to that of the polymerizable resin, a coarse particle fraction, and a fine particle fraction having a mean particle size greater than 0.1 μm and smaller than the mean particle size of the coarse particle fraction. The relative ratio of the coarse particle fraction to the fine particle fraction is in the range from about 12:1 to about 2:1 by volume, the particle size distribution of each fraction is essentially monomodal, and the D(90) of the fine particle fraction is less than or equal to the D(10) of the coarse particle fraction.

CROSS-REFERENCE TO RELATED APPLICATION

Pursuant to 37 C.F.R. §1.78(a)(4), this application claims the benefitof and priority to prior filed Provisional Application Ser. No.61/491,089, filed May 27, 2011, which is expressly incorporated hereinby reference.

FIELD OF THE INVENTION

The present invention relates to resin based materials for use inrestorative dentistry. More specifically, the invention relates to resincomposite materials suitable for simplified placement by the dentalpractitioner, which materials exhibit paste-like viscosity in anundisturbed state and a liquid-like flowable viscosity when subjected tovibration energy.

BACKGROUND OF THE INVENTION

Posterior and anterior tooth restoration is typically achieved byexcavating decayed tooth structure and filling the resulting cavity witha paste-like filling material, which is then hardened by chemical orphotochemical curing processes. Resin based dental restorative materialsare becoming the material of choice by dentists and patients due todesirable esthetic properties. Tooth colored resin based compositematerials are usually composed of dispersions of inorganic fillerparticles in a polymerizable organic resin matrix. Most commonly,especially in direct restorations, the restorative material is cured byexposure to actinic radiation.

Stress bearing restorations, such as those involving the occlusalsurface of posterior teeth, require the use of mechanically strong,highly filled restorative materials to withstand the forces resultingfrom mastication. Such restorative materials are typically highlyviscous, which makes accurate placement of the restorative difficult andhighly technique sensitive. Inadvertently, the cavity may beinsufficiently filled and adaptation of the restorative material to thecavity walls may be incomplete, resulting in gaps between therestoration and the tooth structure, which can lead to increasedsensitivity, intrusion of fluids and bacteria, and can result incontinued tooth decay and premature failure of the restoration. Lesshighly filled, flowable restorative materials, on the other hand,facilitate proper adaptation but lack the required strength for stressbearing restorations. Moreover, since these less highly filled materialstend to flow under their own weight, they cannot be shaped to conform tothe original tooth anatomy.

When using highly filled restorative materials to restore a deep cavity,the material is typically placed incrementally in thin layers. Eachincremental layer is cured individually before placing the subsequentincrement to counteract both polymerization shrinkage stress and lowlight penetration depth and thus incomplete hardening of therestorative. Restoring a tooth using the layering technique is thereforerelatively time consuming and also increases the risk of leaving voidsbetween the layers, which could significantly weaken the restoration.

It is therefore desirable to provide a highly filled, paste-likerestorative material having a high viscosity that can be lowered to aliquid-like flowable consistency when dispensed into a cavity by usingan external stimulus and that the initial paste-like viscosity isrestored upon removal of said external stimulus to facilitate shaping tothe proper contour. Furthermore it is desirable for this material toexert low polymerization shrinkage stress to the restored tooth and toexhibit sufficient depth of cure to enable placement and adequatehardening of fewer but thicker layers of the restorative material.Ideally, the entirety of such a restorative material required to fillthe whole cavity would be placed and hardened in bulk.

It is known that particulate dispersions with high solids content, ofwhich dental restorative composites are examples, typically exhibitshear-thinning and, in some cases, thixotropic behavior and that theirviscosity can be lowered through the action of vibrations, includingsonic or ultrasonic vibrations. For example, U.S. Pat. No. 5,244,933discloses dental compositions having a viscosity too high to be workablefor the intended purpose, which can be rendered workable by exposure tooscillations. When used clinically for restoring a tooth defect,however, such otherwise unworkable materials require the use of specialoscillating equipment throughout each manipulation step, which makestheir use cumbersome and thus presents a disadvantage to thepractitioner. Prior art paste-like resin based restorative materials,including universal composite materials such as Herculite® (Kerr Corp.,Orange, Calif.) can be dispensed at a reduced viscosity through the useof special vibration-assisted dispensers. For example, U.S. Pat. No.7,014,462 discloses a method and a device for introducing a dentalfilling material into a tooth cavity, where the device subjects thefilling material to the action of vibrations as is it injected into thecavity. However, the ease and extent of viscosity reduction of a givenmaterial through the action of sonic or ultrasonic vibrations varies andis highly dependent on the material's composition. Moreover, sincevibration-induced liquefaction is accompanied by heat generation due tointernal friction, simply increasing the power or duration of thevibrating action to improve efficiency can lead to a substantialtemperature rise, which could potentially harm the tooth. For thesepractical reasons, the degree of liquefaction of prior art dentalrestoratives is limited and, particularly for very highly filledmaterials, is insufficient to achieve the liquid-like behavior of aflowable restorative material.

In summary, there is a need to provide a highly filled dentalrestorative material that offers high strength for load bearingrestorations, low polymerization shrinkage, and high depth of cure, yetreadily liquefies to a flowable-like consistency through activation byvibration energy to greatly simplify clinical placement.

SUMMARY OF THE INVENTION

A dental restorative composition is provided, comprising a polymerizableresin, a substantially translucent structural filler, a nanofillerhaving a mean particle size less than 100 nm, and at least onerheology-modifying additive.

In one embodiment, the structural filler has a refractive indexsubstantially similar to that of the polymerizable resin, and comprisesa coarse particle fraction having a first mean particle size, and a fineparticle fraction having a second mean particle size greater than 0.1 μmand smaller than the first mean particle size of the coarse particlefraction. The relative ratio of the coarse particle fraction to the fineparticle fraction is in the range from about 12:1 to about 2:1 byvolume, the particle size distributions of each of the coarse and fineparticle fractions are essentially monomodal, and the D(90) of the fineparticle fraction is less than or equal to the D(10) of the coarseparticle fraction.

In another embodiment, the at least one rheology-modifying additive ispresent in an amount of about 0.1 to about 5 wt. %, the structuralfiller and the polymerizable resin each have a refractive index suchthat RI(filler)−RI(resin)≦0.05, and the structural filler comprises acoarse particle fraction having an essentially monomodal distributionwith a first mean particle size greater than about 3 μm and a D(10)≧1μm, and a fine particle fraction having an essentially monomodaldistribution with a second mean particle size greater than 0.1 μm andless than about 1 μm and a D(90)≦0.9 μm. The relative ratio of thecoarse particle fraction to the fine particle fraction is in the rangefrom about 12:1 to about 2:1 by volume, and the ratio of the structuralfiller to the nanofiller is in the range of about 20:1 to about 10:1 byvolume.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of particle size distribution for a coarse particlefraction of a structural filler for use in a composition of theinvention.

FIG. 2 is a graph of particle size distribution for a fine particlefraction of a structural filler for use in a composition of theinvention.

FIG. 3 is a graph of a blended particle size distribution for thestructural filler combining the coarse particle fraction of FIG. 1 withthe fine particle fraction of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a dental restorative material thatexhibits a paste-like packable viscosity in its undisturbed state thatis readily reduced to a liquid-like flowable viscosity when subjected tovibrations, including sonic and/or ultrasonic vibrations, and exhibitsone or more of the following properties: high loading, high mechanicalstrength sufficient to be suitable for load bearing restorations, lowpolymerization shrinkage, and high depth of cure sufficient to allow thepractitioner to place and adequately harden the material insubstantially deeper incremental layers. Advantageously, the inventivematerial can be used to fill the bulk of a large cavity in one singleincrement, thereby omitting placement and separate hardening of multiplelayers of restorative material as required for prior art dentalcomposites.

In accordance with an embodiment of the invention, the restorativematerial comprises (1) a polymerizable resin; (2) a substantiallytranslucent structural filler having a refractive index substantiallysimilar to that of the polymerizable resin; (3) a nanofiller; (4) one ormore rheology-modifying additives; and optionally (5) other additivesincluding polymerization initiators, dispersants, stabilizers, pigmentsand the like. The structural filler (2) comprises a coarse particlefraction and a fine particle fraction having a mean particle sizesubstantially smaller than the coarse particle fraction, each fractionbeing essentially monomodal having a distribution such that the D(90) ofthe small particle fraction is less than or equal to the D(10) of thelarge particulate fraction, as defined below, and having a relativeratio of coarse particle fraction to fine particle fraction in the rangefrom about 12 to 1 to about 2 to 1 by volume.

The present invention provides a dental restorative resin basedcomposite material having some or all of the following properties: thematerial in its undisturbed state exhibits a paste-like viscosity, whichcan be readily lowered without significant increase in temperature to aliquid-like viscosity when subjected to vibrations including sonicand/or ultrasonic vibrations; high mechanical strength suitable for loadbearing restorations; high filler loading; low polymerization shrinkageand low polymerization shrinkage stress; and high depth of cure whenpolymerized using actinic radiation. By paste-like viscosity it is meantthat the material does not flow under its own weight and can be easilyshaped by the dental practitioner to follow the natural tooth anatomy.By liquid-like viscosity it is meant that the material flows under itsown weight, i.e., is a flowable dental composite as that term isunderstood in the art, such that it intimately conforms to the contoursof the surface to which it is applied without leaving voids between thematerial and the contacting surface. The inventive material can be usedby a dental practitioner to restore a tooth cavity using a greatlysimplified clinical procedure involving significantly fewer manipulatingsteps.

The inventive material comprises a curable resin, preferably apolymerizable resin containing methacrylate monomers. Curing of thecomposite may be achieved by mixing two paste components containing acatalyst and accelerator, respectively, or by a photopolymerizationprocess wherein the resins are cured when exposed to actinic radiation,such as blue visible light. Photopolymerizable resins containingmonomers other than methacrylates may be used in the present invention,as may be appreciated by those skilled in the art, such as cationicallyphotocurable oxiranes, for example.

To provide the inventive material exhibiting the desired rheology, inparticular to achieve a pronounced reduction in viscosity upon beingsubjected to vibrations, the material advantageously exhibits a highdegree of shear thinning. Without being bound by theory, the extent ofshear thinning can be maximized by employing an essentially uniformlysized structural filler. As can be appreciated by a person skilled inthe art, such a material exhibits less desirable handling propertiesincluding a tendency to flow under its own weight, which would makeshaping by the practitioner very difficult. Furthermore, by using auniformly sized filler, the desired high degree of loading cannot beachieved, since a high, unfilled interstitial volume between the fillerparticles remains.

It has now been found that addition of at least one rheology modifyingadditive to a composition comprising a polymerizable resin and a coarsestructural filler having an essentially monomodal particle sizedistribution significantly improves handling properties, such that apaste-like viscosity is achieved and the material maintains its shapewithout flowing under its own weight and can thus be easily shaped.Monomodal particle size distribution as used herein refers to acollection of particles whose size distribution curve (particle count asa function of particle size) displays one single maximum and, whenparticle size is plotted on a logarithmic scale, shows an approximatelylognormal distribution. Particle size distribution is measured by knownmethods, including but not limited to dynamic light scattering methods.Advantageously, addition of at least one rheology modifying additiveimparts thixotropic properties to the composite paste, which has beenfound to improve material flow when being subjected to sonic and/orultrasonic vibrations.

It has further been found that addition of a fine particle filler,referred to as the fine particle fraction of the structural filler,having an essentially monomodal distribution and having a mean particlesize substantially smaller than the coarse particle fraction, such thatthe D(90) of the fine particle fraction is less than or equal to theD(10) of the coarse particle fraction, greatly increases the degree ofloading by filling the interstitial spaces. D(90) is defined as thediameter where 90% by volume of the particles within the distributionhave a smaller diameter. D(10) is defined as the diameter where 10% byvolume of the particles within the distribution have a smaller diameter.FIG. 1 graphically depicts a particle distribution of an exemplarycoarse particle fraction having a mean particle size of about 5.3 μm, aD10 of about 1.58 μm, and a range of particle sizes of about 0.15-22 μm.FIG. 2 graphically depicts a particle distribution of an exemplary fineparticle fraction that may be used with the coarse particle fraction ofFIG. 1, the fine particle fraction having a mean particle size of about0.52 μm, a D90 of about 0.75 μm, and a range of particle sizes of about0.15-1.5 μm. FIG. 3 graphically depicts the blended particledistribution for the structural filler combining the coarse particlefraction of FIG. 1 with the fine particle fraction of FIG. 2. In anotherexample, a coarse particle fraction having a mean particle size of about5 μm, a D10 of about 1 μm, a D90 of about 11 and a range of particlesizes of about 0.13-25 μm may be used with the small particle fractionof FIG. 2.

It is advantageous to increase the loading, i.e., the fraction of solidsdispersed within the hardenable resin to an extent that upon hardeningof the mixture a volumetric shrinkage of less than about 1.8% isachieved. In one embodiment, the fine particle fraction is added at aratio of coarse to fine particles of about 12 to 1 to about 2 to 1 byvolume, since this range still provides the desired high degree of shearthinning. Coarse particle fraction refers to the collection of fillerparticles falling within the size distribution having the larger meanparticle size and does not infer any particular absolute particle sizeor range. Fine particle fraction refers to the collection of fillerparticles falling within the size distribution having the smaller meanparticle size and does not infer any particular absolute particle sizeor range; however, for practical reasons, the mean particle size of thefine particle fraction is greater than 0.1 μm. The combined coarse andfine particle fractions are referred to herein as the structural filler.

By way of example only, and not limitation, the coarse particle fractionmay have a mean particle size greater than about 3 μm with the sizedistribution of particles including a D(10) of no less than about 1 μm(i.e., ≧1 μm). Additionally, the fine particle fraction may have a meanparticle size greater than about 0.1 μm and less than about 1 μm withthe size distribution of particles including a D(90) of no greater thanabout 0.9 μm. By way of further example, the structural filler mayinclude a coarse particle fraction having a mean particle size in therange of about 4-7 μm with the size distribution of particles includinga D(10) of no less than about 1.2 μm and a fine particle fraction havinga mean particle size of about 0.3-0.7 μm with the size distribution ofparticles including a D(90) of no greater than about 0.9 μm.

The structural filler exhibits a particle surface that is sufficientlyhydrophobic to increase compatibility with the resin and at the sametime minimizes particle-particle interactions. It is particularly usefulfor the structural filler particles to exhibit a refractive indexsubstantially similar to that of the polymerizable resin, such thatRI(filler)−RI(resin)≦ about 0.05 and that the filler is substantiallytranslucent. RI(filler) and RI(resin) are the refractive indexes of thefiller and the resin, respectively. The translucency of the filler isdetermined by dispersing 75 parts by weight of filler into 25 parts byweight of hardenable resin where RI(filler)−RI(resin)≦0.05, hardeningthe paste, and measuring the opacity of the resulting mixture. By havinga substantially translucent filler it is meant that a mixture preparedas described is less than 45% opaque. Both properties provide for highdepth of cure when the material is hardened using blue visible light.

The coarse particle fraction may be selected from, but not limited to,borosilicate glass, barium magnesium aluminosilicate glass, bariumaluminosilicate glass, amorphous silica, zirconium silicate, titaniumsilicate, barium oxide, quartz, alumina and other inorganic oxideparticles. A chemical sol gel process may be used to manufacture thefiller, or in an exemplary embodiment, a filler may be ground to thesize range by a commination step. The coarse filler particles may besurface treated with gamma-methacryloxypropyl trimethoxysilane or, morepreferably with a long-chain polymerizable silane having the generalformula:

wherein R¹ is a (meth)acrylate group functionalized radical; n is aninteger; Z is selected from —C(═O)—, —C(═S)—, —C(═O)—O—, —C(═O)—S—,—C(═S)—O—, —C(═S)—S—, —C(═O)—NR⁵—, —C(═S)—NR⁵—, and —C(R⁶)₂—; Q isoptional and represents an alkylene spacer, wherein the succession ofcarbon atoms may be interrupted by heteroatoms including N, O, or S; R²is selected from halogen or alkoxy groups; R³, R⁴ independently areselected from hydrogen, halogen, alkyl, aryl, alkoxy, and aryloxy; R⁵and R⁶ independently are hydrogen, alkyl or aryl radicals.

The coarse particle fraction may also be selected from a prepolymerizedfiller. For example, U.S. Pat. No. 6,890,968, which is incorporatedherein by reference, discloses the preparation and use of prepolymerizedfiller particles suitable for the present invention. The prepolymerizedfiller particles are prepared by mixing an inorganic filler with anorganic polymerizable resin and curing the mixture. The cured mixture isthen ground to a desired size. The ground prepolymerized filler may befurther classified to separate the desired coarse particle fraction outof the polydisperse ground prepolymerized filler to provide theprepolymerized filler with an essentially monomodal particle sizedistribution. For example, the coarse particle fraction may have a meanparticle size of about 30-70 μm with a D(15) of 10 μm, as disclosed inthe '968 patent. Optionally, the classified ground prepolymerized fillermay be surface treated with gamma-methacryloxypropyl trimethoxysilane orother suitable silanes.

The fine particle fraction may be selected from, but not limited to,borosilicate glass, barium magnesium aluminosilicate glass, bariumaluminosilicate glass, amorphous silica, zirconium silicate, titaniumsilicate, barium oxide, quartz, alumina and other inorganic oxideparticles and may be of the same or different material as the coarseparticle fraction. A chemical sol gel process may be used to manufacturethe filler, or in an exemplary embodiment, a filler may be ground to thedesired size range by an extensive comminution step. The fine fillerparticles may be surface treated with gamma-methacryloxypropyltrimethoxysilane. An example for a fine particle filler particularlysuitable for the present invention is disclosed in U.S. Pat. No.6,121,344, which is incorporated herein by reference. For example, thefine particle fraction may have a mean particle size of 0.62 μm and aD(90) of 0.82 μm as prepared by Method A disclosed therein, or a meanparticle size of 0.47 μm and a D(90) of 0.76 μm as prepared by Method Bdisclosed therein, or a mean particle size of 0.36 μm and a D(90) of0.61 μm as prepared by Method C disclosed therein.

To further increase the loading and to increase material strength, theinventive material also comprises a nanofiller having a mean particlesize of less than 100 nm. Generally, the particle size distribution fora nanofiller is less than 100 nm, with the largest particles beingsmaller than the smallest particles of the fine particle fraction of thestructural filler. In an exemplary embodiment, the nanofiller is addedat a ratio of structural filler to nanofiller of about 20 to 1 to about10 to 1 by volume. Structural filler refers to the combined coarse andfine particle fractions. In an exemplary embodiment, the nanofillercomprises essentially discrete, non-agglomerated particles. Colloidalsilica is exemplary. Fumed silica is excluded herein from the scope ofnanofillers.

The rheology-modifying additive may comprise an inorganic rheologymodifier, an organic rheology modifier, or particularly advantageously acombination of both. One particularly suitable inorganic rheologymodifier is pyrogenic (fumed) silica. Thus, fumed silica, for purposesof this invention, falls within the category of a rheology-modifyingadditive, and is not considered a nanofiller. Exemplary organic rheologymodifiers are small molecules or polymers capable of forming strongnon-covalent intermolecular interactions such as ionic or hydrogenbonding. Specific examples of organic modifiers include those disclosedin U.S. Pat. No. 6,395,803, which is incorporated herein by reference.Alkylamide compounds, in particular, as disclosed therein are useful inthe present invention. In general, as set forth in the '803 patent,alkylamides of the general formula RCONHR′, where R is an alkyl oralkylidine group and R′ is an alkyl group, are useful asrheology-modifying additives in dental restorative compositions,particularly where an R or R′ alkyl group has 4 or more carbons andpreferably 10 or more. For example, the rheology-modifying additive maybe one or more of the following: (1) a hydroxyfunctional polycarboxylicacid amide; (2) the reaction product of from about 15 to 75 parts byweight of one or more liquid polyalkoxylated nitrogen-containingcompounds containing more than one hydroxyl group and which also containa pendant aliphatic radical of 6 to 40 carbon atoms selected from thegroup consisting of tertiary amines and amides of secondary amines, fromabout 8 to 90 parts by weight of one or more polycarboxylic acids, andfrom about 0.5 to 20 parts by weight of one or more liquid diamines of amolecular weight (weight average) of about 2000 or less, wherein thereaction is continued until the acid value is within the range of 5 to14 and the amine value is within the range of 42 to 84; (3)trialkylamidocyclohexanes, such as trialkyl cis-1,3,5-cyclohexanetricarboxamides; (4) carbobenzyloxy-containing alkylamides, suchas N-carbobenzyloxy-L-isoleucylaminooctadecane; (5) L-valine-containingbenzenedicarbonyl derivatives, such as N,N′terephthaloyl-bis(L-valylaminododecane) and N,N′terephthaloyl-bis(L-valylaminooctadecane); and derivatives oftrans-1,2-diaminocyclohexane, such astrans-1,2-bis(dodecylamido)cyclohexane, the polymerizable derivative(1R,2R)-trans-1,2-bis(2-(methacryloyloxy)ethylsuccinamido) cyclohexaneand trans-1,2-bis(ureido)cyclohexane. It is believed that a rheologymodifier of the type in example (1) above may be obtained from BYKChemie USA, Wallingford, Conn. under the trade name BYK®-405, and that arheology modifier of the type in example 2 above may be obtained fromRheox Corporation, Hightstown, N.J. under the trade name Thixatrol®VF-10.

In one embodiment, the rheology modifying additives may be present in anamount of about 0.1 to about 5 percent by weight of the total pastemixture. By way of example, and not limitation, the rheology-modifyingadditive may include a hydroxyfunctional polycarboxylic acid amideaccording to the Formula (1) provided in the '803 patent, which may bepresent in an amount of about 0.1-0.7 percent by weight. Fumed silicamay be added in addition to the organic rheology modifier, for examplein an amount of about 1-3 wt. %. More than one fumed silica may be used,for example each with a different average particle size, for example, a20 nm fumed silica and a 40 nm fumed silica, and/or a combination ofhydrophilic and hydrophobic fumed silicas.

The dental restorative compositions of the invention may further includea dispersant, for example, as described in the '803 patent. Morespecifically, phosphoric acid esters (including mono-, di- andtri-esters), such as polymerizable phosphate polyesters, may be used.Particularly, phosphoric acid esters useful in the present invention areselected from the following: a) a phosphoric acid ester containing acarboxylic acid ester group and an ether group, and b) a phosphoric acidester containing a carboxylic acid ester group and not containing anether group. One example of a dispersant for use in the presentinvention may be obtained from BYK Chemie USA, Wallingford, Conn. underthe trade name Disperbyk®-111. Other examples includepolycaprolactone-modified methacrylate monophosphates, such aspolycaprolactone-modified hydroxyethyl methacrylate phosphate,polycaprolactone-modified hydroxyethyl acrylate phosphate,polycaprolactone-modified polypropylene glycomethacrylate phosphate,polycaprolactone-modified glycerol dimethacrylate phosphate,polycaprolactone-modified dipentaerythritol pentaacrylate phosphate, andpolycaprolactone-modified polyethylene glycol monomethacrylatephosphate. By way of example, a dispersant may be present in an amountof 5 weight percent or less, such as 0.5-3.5 weight percent.

EXAMPLES Compressive Strength (CS) Test

The specimens were prepared by condensing the dental restorativecomposition, in paste form (referred to simply as “the paste”), into astainless-steel mold with a dimension of 4 mm (diameter)×3 mm (height),and then photo-curing the paste with a Demetron Optilux™ 501 curinglight (Kerr Corp.) for 30-seconds from each side. The cured disk wasremoved from the mold and conditioned in 37° C. water for 24 hoursbefore subjecting to mechanical testing on an Instron Universal Tester(Model 4202) in compression mode with a crosshead speed of 0.50mm/minute. The peak load at which the specimen broke was used tocalculate the CS expressed in MPa unit. Six specimens were tested foreach formula.

Diametral Tensile Strength (DTS) Test

The specimens were prepared by condensing the paste into astainless-steel mold with a dimension of 6 mm (diameter)×3 mm (height),and then photo-curing the paste with a Demetron Optilux™ 501 curinglight (Kerr Corp.) for 30-seconds from each side. The cured disk wasremoved from the mold and conditioned in 37° C. water for 24 hoursbefore subjecting to mechanical testing on an Instron Universal Tester(Model 4202) in compression mode with a crosshead speed of 10 mm/minute.The load was applied in the diameter direction in compression mode. Thepeak load at which the specimen broke was used to calculate the DTSexpressed in MPa unit. Six specimens were tested for each formula.

Flexural Strength (FS) and Young's Modulus (E) Tests

FS and E were measured from the same flexural test according to ISO 4049standard. The specimens were prepared by condensing the paste into astainless-steel mold with a dimension of 2 mm×2 mm×25 mm, and thenphoto-cured from both sides. The cured disk was removed from the moldand conditioned in 37° C. water for 24 hours before subjecting tomechanical testing on an Instron Universal Tester (Model 4202) in3-point bending mode with a crosshead speed of 0.5 mm/minute. The peakload at which the specimen broke was used to calculate the FS expressedin MPa unit. E was obtained from the slope of the stress-strain curve inthe initial linear region. Five specimens were tested for each formula.

Volumetric Polymerization Shrinkage (VPS)

VPS was calculated based on the measured densities of the materialbefore and after light-curing with the Demetron Optilux™ 501 curinglight for 60 seconds (30 seconds each side). The density was measuredusing the buoyancy method in de-ionized water.

Complex Viscosity (eta*) and Loss Tangent (tan δ)

Rheological properties were determined using a stress-controlled dynamicoscillatory Rheometer (DSR-200, Rheometrics Scientific, Piscataway,N.J.) in parallel plate configuration with plate diameter of 10 mm and agap of 1.0 mm at a constant frequency of 1.0 Hz. A low stress of 50 Pawas applied for 300 s and complex viscosity (eta*) in Pa·s and losstangent were recorded. These values are indicative of the viscosity inthe undisturbed state. The samples were subsequently subjected to highstress of 1000 Pa for 30 s and complex viscosity and loss tangent wererecorded. These values are indicative of the viscosity when sonic orultrasonic vibrations are applied. A Loss Tangent of less than 1indicates solid or elastic properties that are desirable for theundisturbed state for purposes of shaping the composition to the toothanatomy without the composition flowing under its own weight. A LossTangent of greater than 1 indicates liquid-like properties that aredesirable for flowing the composition through a delivery means.

Abbreviations for Materials Used in all Examples:

Bis-GMA 2,2-bis[4-(2-hydroxy-3-methacryloylpropoxy)-phenyl]- propane CQcamphorquinone EDMAB ethyl 4-(N,N-dimethylamino) benzoate EBPADMA-ethoxylated bisphenol A dimethacrylate with 2.5 moles of 2.5EO ethyleneoxide EBPADMA- ethoxylated bisphenol A dimethacrylate with 6 moles of6EO ethylene oxide MACL-TES polymerizable polyester triethoxysilaneHEMA- polymerizable phosphate polyester: polycaprolactone- CL5-Pmodified hydroxyethylmethacrylate with 5:1 mole ratio ofcaprolactone:starting material MEHQ 4-methoxyphenol R-202 fumed silica,Aerosil R-202; 14 nm primary particles; aggregated ST-OX-50 fumedsilica, OX-50 surface treated with γ-methacryloyloxypropyltrimethoxysilane; 40 nm primary particles; aggregated ST-BAS-0.5Bariumaluminoborosilicate filler, mean particle size of 0.5 μm, D(90) of0.8 μm, surface treated with γ- methacryloyloxypropyl trimethoxysilaneST-BAS-0.7 Bariumaluminoborosilicate filler, mean particle size of 0.7μm, D(90) of 1.1 μm, surface treated with γ- methacryloyloxypropyltrimethoxysilane ST-BAS-4G Bariumaluminoborosilicate filler, meanparticle size of 4 μm, D(10) of 1.5 μm, surface treated with γ-methacryloyloxypropyl trimethoxysilane ST-BAS-4PBariumaluminoborosilicate filler, mean particle size of 4 μm, D(10) of1.5 μm, surface treated with MACL-TES UV-92-hydroxy-4-methoxybenzophenone TEGDMA TriethyleneglycoldimethacrylateBYK-405 hydroxyfunctional polycarboxylic acid amide

In all the examples for making the single-part, light-curablerestorative material compositions, a homogeneous resin mixture was madefirst by mixing all monomers with initiators and additives that aresoluble in the resin mixture. The resin composition is provided inTable 1. Then the resin mixture was further blended together withvarious fillers to make the restorative composition, in paste form.Unless otherwise indicated, all parts and percentages are by weight inall examples.

TABLE 1 Resin Composition BisGMA  9.9 weight % TEGDMA  4.9 weight %EBPADMA-2.5EO 24.7 weight % EBPADMA-6EO 59.2 weight % UV-9 0.49 weight %MEHQ 0.06 weight % CQ 0.23 weight % EDMAB 0.49 weight % Total 100

The compositions used for all examples and their testing results arelisted in Table 2.

TABLE 2 Dental Restorative Compositions A B C D E F wt % wt % wt % wt %wt % wt % Resin 17.8 17.8 14.9 19.8 19.3 15.2 HEMA-CL5-P 1.0 1.0 1.0 1.01.0 1.0 Colloidal silica (80 nm, 4.0 4.0 4.0 4.0 4.0 mostlynon-agglomerated) BYK-405 0.2 0.2 0.2 0.2 0.2 Fumed silica 0.5 0.5 1.20.5 0.5 ST-BAS-0.5 15.1 ST-BAS-0.7 13.5 60.5 15.0 15.8 ST-BAS-4G 76.563.0 14.0 64.0 64.0 ST-BAS-4P 63.6 Total loading (wt %) 81.0 81.0 83.979.0 79.5 83.8 TESTING RESULTS CS (MPa) 258 277 363 360 256 299 FS (MPa)105 129 159 139 117 132 E (GPa) 13.7 12.6 12.9 11.6 12.4 15.0 DTS (MPa)55 VPS (%) 2.1 1.7 2.2 1.9 eta* (50 Pa stress) (Pa · s) 20712 3053 380413460 1574 74 tanδ (50 Pa stress) 0.57 0.43 0.35 0.34 0.61 2.42 eta*(1000 Pa stress) (Pa · s) 1262 618 615 3737 444 364 tanδ (1000 Pastress) 1.03 1.34 1.09 0.49 1.45 1.50

Example A illustrates a composition having a large particle fraction andnanofiller, but lacking a small particle fraction. While the compositionundergoes shear thinning, as indicated by the reversal in the LossTangent, the Complex Viscosity is so high in the undisturbed state thatit cannot be easily manipulated and may be considered too hard to bepractically useful, and is so high in the disturbed state that it maynot or will not easily flow through a desired delivery means.

Example B illustrates a composition nearly identical to that of ExampleA, but with a minor portion of the large particle fraction substitutedby a small particle fraction in accordance with the invention. The LossTangent values indicate shear thinning upon vibration and the ComplexViscosities indicate good shaping and flow properties for the twostates.

Example C illustrates a composition similar to Example B, but withslightly higher total filler loading, which is desirable, and anincreased content of the inorganic rheology modifier to enable thehigher loading. Even with the higher loading, the Loss Tangent valuesindicate shear thinning upon vibration and the Complex Viscositiesindicate good shaping and flow properties for the two states.Additionally, low volumetric shrinkage is obtained.

Example D illustrates a composition similar to Example B, but with thequantity of small and large particle fractions essentially reversed,i.e., higher quantity of small particle fraction and lower quantity oflarge particle fraction. The composition exhibited little to no shearthinning, i.e., it did not transition to a liquid-like state uponapplication of vibration.

Example E illustrates a composition similar to Example B, but lackingthe nanofiller, i.e., the non-agglomerated colloidal silica. Thecomposition still undergoes shear thinning upon the application ofvibration, but the Complex Viscosities indicate that the material is toosoft in the undisturbed state such that it will begin to flow under itsown weight and will be difficult to shape.

Example F illustrates a highly loaded composition similar to Example C,but lacking both the organic and inorganic rheology modifiers. Thematerial was unacceptably soft in the undisturbed state such that itflows under its own weight, and actually experiences shear thickeningupon the application of vibration.

While the present invention has been illustrated by the description ofone or more embodiments thereof, and while the embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. The invention in its broader aspects is thereforenot limited to the specific details and method and illustrative examplesshown and described. Accordingly, departures may be made from suchdetails without departing from the scope of the general inventiveconcept.

What is claimed is:
 1. A dental restorative composition, comprising: apolymerizable resin; a substantially translucent inorganic structuralfiller having a refractive index substantially similar to that of thepolymerizable resin, wherein the structural filler consists of: aninorganic coarse particle fraction having a first mean particle size inthe range of about 4-7 μm and a D(10)≧1.2 μm, an inorganic fine particlefraction having a second mean particle size greater than 0.1 μm andsmaller than the first mean particle size of the coarse particlefraction and a D(90)≦0.9 μm, and optionally a silane surface treatmenton the inorganic coarse particle fraction or the inorganic fine particlefraction or both, wherein a relative ratio of the coarse particlefraction to the fine particle fraction is in the range from about 12:1to about 2:1 by volume, and the particle size distributions of each ofthe coarse and fine particle fractions are essentially monomodal; ananofiller having a mean particle size less than 100 nm; and at leastone rheology-modifying additive.
 2. The composition of claim 1, whereinthe at least one rheology-modifying additive comprises an inorganicrheology modifier.
 3. The composition of claim 2, wherein the at leastone rheology-modifying additive is fumed silica.
 4. The composition ofclaim 2, wherein the at least one rheology-modifying additive furthercomprises an organic rheology modifier.
 5. The composition of claim 4,wherein the organic rheology modifier is an alkylamide of the generalformula RCONHR′, where R is an alkyl or alkylidine group and R′ is analkyl group.
 6. The composition of claim 1, further comprising apolymerizable phosphate polyester dispersant.
 7. The composition ofclaim 1, wherein the at least one rheology-modifying additive is presentin an amount of about 0.1 to about 5 wt. %.
 8. The composition of claim1, wherein the nanofiller is essentially non-agglomerated colloidalsilica.
 9. The composition of claim 1, wherein, for the fine particlefraction, the second mean particle size is in the range of about 0.3-0.7μm.
 10. The composition of claim 1, wherein the refractive index of thestructural filler minus the refractive index of the polymerizable resinis less than or equal to about 0.05.
 11. The composition of claim 1,wherein the ratio of the structural filler to the nanofiller is in therange of about 20:1 to about 10:1 by volume.
 12. The composition ofclaim 1, wherein the viscosity is paste-like in an undisturbed state,and the composition undergoes shear-thinning to reduce the viscosity toliquid-like when subjected to sonic and/or ultrasonic vibration.
 13. Thecomposition of claim 1, wherein, upon curing, the composition exhibitsless than about 1.8% volumetric shrinkage.
 14. A dental restorativecomposition, comprising: a polymerizable resin having a refractiveindex, RI(resin); a substantially translucent inorganic structuralfiller having a refractive index, RI(filler), whereinRI(filler)−RI(resin)≦0.05, and wherein the structural filler consistsof: an inorganic coarse particle fraction having an essentiallymonomodal distribution with a first mean particle size in the range ofabout 4-7 μm and a D(10)≧1.2 μm, an inorganic fine particle fractionhaving an essentially monomodal distribution with a second mean particlesize greater than 0.1 μm and less than about 1 μm and a D(90)≦0.9 μm,and optionally a silane surface treatment on the inorganic coarseparticle fraction or the inorganic fine particle fraction or both,wherein a relative ratio of the coarse particle fraction to the fineparticle fraction is in the range from about 12:1 to about 2:1 byvolume; a nanofiller having a mean particle size less than 100 nm,wherein the ratio of the structural filler to the nanofiller is in therange of about 20:1 to about 10:1 by volume; and at least onerheology-modifying additive in an amount of about 0.1 to about 5 wt. %.15. The composition of claim 14, wherein the at least onerheology-modifying additive comprises an inorganic rheology modifier andan organic rheology modifier.
 16. The composition of claim 14, whereinthe at least one rheology-modifying additive includes fumed silica andan alkylamide.
 17. The composition of claim 14, wherein, for the fineparticle fraction, the second mean particle size is in the range ofabout 0.3-0.7 μm.
 18. The composition of claim 14, wherein the viscosityis paste-like in an undisturbed state, and the composition undergoesshear-thinning to reduce the viscosity to liquid-like when subjected tosonic and/or ultrasonic vibration.
 19. The composition of claim 14,wherein, upon curing, the composition exhibits less than about 1.8%volumetric shrinkage.